Electrodialysis apparatus

ABSTRACT

An integral, monolithic frame-membrane is disclosed, such frame-membrane having a semi-permeable membrane portion and integral therewith a frame portion, the frame portion having one or more cavities, each cavity juxtaposed to the membrane portion, each cavity having at least one fluid entrance conduit communicating with an entrance manifold aperture and at least one fluid exit conduit communicating with an exit manifold aperture. The integral, monolithic frame-membrane may be used in apparatus for carrying out gas-separation; microfiltration; ultrafiltration; nanofiltration; reverse osmosis (i.e. hyperfiltration); diffusion dialysis; Donnan dialysis; electrodialysis (including filled-cell electrodialysis; i.e. electrodeionization); pervaporation; piezodialysis; membrane distillation; osmosis; thermo osmosis; and electrolysis with membranes. Also disclosed are pillows prepared from ion exchanging films or fabrics (which may be porous or non-porous), the pillows filled with ion exchange structures such as beads, fibers, fabrics or rods. The pillows may be used with the integral, monolithic frame-membranes, with separate frames and membranes or with frames only.

This application is a continuation of application Ser. No. 08/888,151filed Jul. 3, 1997 issued as U.S. Pat. No. 5,891,328, which is acontinuation of application Ser. No. 08/410,423 filed Mar. 23, 1995,abandoned.

BACKGROUND OF THE INVENTION

In "multi-compartment electrodialysis" ("ED") many ion exchange ("IX")membranes are arranged between a single pair of electrodes. Themembranes are of two types: cation exchange membranes ("CXM") and anionexchange membranes ("AXM"). The CXM are relatively permeable topositively charged, low molecular weight ions and relatively impermeableto negatively charged ions and to high molecular weight neutralmolecules whereas AXM are relatively permeable to negatively charged,low molecular weight ions and relatively impermeable to positivelycharged ions (and also to high molecular weight neutral molecules). TheCXM and AXM alternate between the above mentioned single pair ofelectrodes. Spaces are left between the membranes through which arepassed aqueous solutions. When a direct electric current is passedbetween the electrodes, positively charged ions in the solutions("cations", generally metallic ions such as sodium, magnesium, calcium)are pulled toward the negatively charged electrode ("cathode"). Smallcations easily pass through CXM but not through AXM. Simultaneouslynegatively charged ions in the solutions ("anions", generallynon-metallic ions such as chloride, nitrate, bicarbonate, fluoride,sulfate) are pulled toward the positively charged electrode ("anode").Small anions easily pass through AXM but not through CXM. As a result,spaces which are on the cathode side of AXM (on the anode side of CXM)are at least partially deionized by the direct electric current. Suchspaces are generally called "diluting" spaces. Spaces which are on theanode side of AXM (on the cathode side of CXM) accumulate the ionsremoved from the diluting spaces. Such enriched spaces are generallycalled "concentrating" spaces. ED is made continuous by flowing thesolutions between the membranes. In such case the spaces must beenclosed by frames ("spacers") to prevent mixing of the solution in thediluting spaces ("dilute solution" or "dilute") with that in theconcentrating spaces ("concentrate solution" or "concentrate"). Holesare provided in the membranes registering with similar holes in theframes thereby forming (internal) manifolds to distribute the solutionsto the appropriate spaces (cavities in the frames) and to collectseparately the dilute and concentrate solutions. Channels in the frameswhich enclose the concentrating spaces connect such spaces with theconcentrate solution manifolds, and channels in the frames which enclosethe diluting spaces connect the latter spaces with the dilute solutionmanifolds. The channels and/or frames are arranged (designed) so thatthe flows of solutions are uniform over the surfaces of the membranes.Regions in the diluting spaces in which the solution velocity is lowerthan average will be more highly deionized than the average extent ofdeionization. Regions in the concentrating spaces in which solutionvelocity is lower than average will be more enriched (concentrated) thanaverage. If the solution being deionized contains sparingly solublesalts (such as calcium sulfate or calcium bicarbonate) then such saltsmay precipitate on or in those membrane surfaces in contact with regionsin the concentrating spaces having velocities lower than average.

Roughly 95% of the direct electric current passing through CXM iscarried by cations passing from the diluting spaces to the concentratingspaces (the remaining 5% carried by anions passing through the CXM fromthe concentrating spaces to the diluting spaces). The fraction of thecurrent carried by a given ion is referred to as the "transport number".In the above case the transport number of CXM for cations isapproximately 0.95. Similarly the transport number of AXM for anions isapproximately 0.95 (and the transport number for cations passing throughthe AXM from the concentrating spaces to the diluting spaces is about0.05). In aqueous solutions of mineral salts, the transport number forboth anions and cations is roughly 0.5, that is roughly half the currentis carried by anions and half by cations. For example, in the case ofaqueous solutions of sodium chloride, 40% of the electric current iscarried by positively charged sodium ions (i.e. transport number 0.4)and 60% by negatively charged chloride ions. Hence while 95% of theelectric current is carried by sodium ions through a CXM (in the case ofsodium chloride solutions) only 40% of the current is carried to the CXMby sodium ions. At the interface between the dilute solution and the CXMthere is therefore a deficit in sodium ion transport equivalent to95-40=55% of the current. Such deficit leads rapidly (in roughly onesecond) to a reduction in the sodium chloride concentration at the abovementioned interface to such a value that the missing 55% of the currentis brought to the interface by diffusion. A mass balance leads to##EQU1## where "i" is the current density in amperes per cm², t is thetransport number for sodium ions in the CXM (0.95), t is the transportnumber for sodium ions in aqueous solutions of sodium chloride (0.40), Dis the diffusion coefficient (in cm² per second) for sodium chloride(not sodium ions) in aqueous solution (1.36×10⁻⁵ at 18° C., 64° F.), Fis the quantity of current required to transport one gram-equivalent ofany ion at a transport number of exactly 1 (96, 480 amperes pergram-equivalent per second, called Faraday's constant), ##EQU2## is theconcentration gradient built up because of the deficit in electricaltransport of sodium ions, C_(m) is the concentration (ingram-equivalents per cm³) at the above mentioned interfaces, δ is thedistance (in cm) from the interface to the plane in the aqueous solution(parallel to the membrane) where the concentration is C. If the flow isin the streamline (laminar flow) region then δ is half the distance fromthe CXM through the diluting space to the AXM. Even when the generalflow is not in the streamline region, a layer of solution adjacent tothe CXM membrane remains in the streamline region, in which case δ isthe thickness of such layer and C is the concentration outside suchlayer, essentially the bulk concentration. The above mass balanceequation may be rearranged to: ##EQU3## where ##EQU4## is the quantityof sodium ions (in gram-equivalents per second per cm²) transported by acurrent having a density of i amperes per cm². Inspection of thisequation shows that ##EQU5## increases as D (the specific rate ofdiffusion) increases. D is inversely proportional to the viscosity ofthe solution and directly proportional to the absolute temperature andtherefore in the case of aqueous solutions increases at a compound rateof about 2.25% per ° C. Therefore there is an advantage to operating atelevated temperatures. The electrical energy consumption (inwatt-seconds per gram-equivalent of sodium ion removed through a CXM) isiRF/t where (as mentioned above) i is in amperes per cm², F is thequantity of current required to remove 1 gram-equivalent per second ofsodium ion through a CXM having a transport number of exactly 1 (96,480amperes per gram-equivalent per second), t is the actual transportnumber of the CXM and R is the sum of the electrical resistances per cm²of one CXM, one AXM, one diluting space and one concentrating space. Rdecreases at a compound rate of about 2% per ° C. (i.e. the same rate atwhich the viscosity of water decreases), increasing the importance ofoperating at elevated temperatures. Almost all of the electrical energyconsumption goes into heating the dilute and concentrate solutions.Therefore it is often practical to warm those solutions by recuperativeheat exchangers, transferring heat from the effluent solutions to theinfluent solutions. If the electrical energy used in ED is generatedlocally (e.g. by a diesel electric generator or a packaged streamelectric generator) then the waste heat may be used to elevate thetemperatures of the influent solutions at least enough to makerecuperative heat exchange practical.

Further inspection of the above equation (2) shows that it/F increasesas t/(t-t) increases as illustrated in the following table:

    ______________________________________                                                         Relative    Relative                                                                            Relative                                   t        t/(t - t)                                                                             it/F        i     Energy                                     ______________________________________                                        0.95     1.73    1.00        1.00  1.00                                       0.85     1.89    1.09        1.22  1.37                                       0.75     2.14    1.24        1.57  1.99                                       0.65     2.60    1.51        2.20  3.22                                       0.55     3.67    2.12        3.67  6.33                                       0.45     9.00    5.21        11.00 23.2                                       ______________________________________                                    

The third column in the above table ("Relative it/F") is the relativeamount of sodium ion removed per cm² per second, other things on theright hand side of above equation (2) being equal (i.e. C, C_(m), D(which is a constant for sodium chloride dependent only on temperature)and δ). The fourth column ("Relative i") is the relative current density(in amperes per cm²) corresponding to t and it/F while the last column("Relative Energy") is the relative energy (watt-seconds pergram-equivalent sodium ion removed) other things on the right hand sideof equation (2) being equal. It will be seen that, except for the casewhere very few gram-equivalents of sodium must be removed, it may not bepractical to increase it/F (the rate of removal of sodium ion) byincreasing t/(t-t). (Water having 584 ppm sodium chloride has 10gram-equivalents of sodium per cubic meter (1 cubic meter=1 metrictonne=264 U.S. gallons)).

Inspection of equation (2) also shows that it/F (the rate of removal ofsodium ion in gram-equivalents per cm² per second) increases as δdecreases. (δ is the distance (in cm) from the interface between the CXMand the solution to the plane in the aqueous solution (parallel to themembrane) where the concentration is C). In many respects, δ is only anadjustable constant which correlates the data and cannot otherwise bemeasured. It correlates with a similar "δ" (but is not the same as)derived from liquid momentum transfer to the membrane (viscosity lossdue to friction at the membrane). It also correlates with (but is notthe same as) a similar 6 derived from heat transfer from solution to themembrane. δ, for diffusion of sodium chloride to a membrane, is about0.05 cm for non flowing solutions (apparently due to gravitationalconvection caused by density differences, in turn caused byconcentration and temperature differences). If the distance from the CXMthrough the dilute solution to the AXM is 0.01 cm then, even when theflow of dilute solution is in the streamline (laminar flow) region, δwill be 0.005 cm. (Such CXM/AXM separation seems never to have beentried; no doubt there are interesting engineering problems associatedwith such small separation but there are no fundamental reasons why suchis not possible). Values of δ of 0.005 cm are achieved in tortuous pathspacers in use commercially from 1954. Such spacers have thicknesses of0.1 cm (the distance from the CXM to the AXM is 0.1 cm) and haverectangular barriers 0.05 cm thick spaced about 1.4 cm from each other.The barriers alternate from the CXM side to the AXM side. Other spacersare in use or have been in use which use no barriers at all or usebarriers provided by screens, expanded plastic sheet or perforated andcorrugated plastic sheet. Non-woven screens having a close spacingbetween strands and oriented to make the flow direction change veryfrequently by 90° angles, easily result in δ of about 0.001 cm (andtherefore in much increased it/F, other things in equation (2) beingequal). Spacers may be rated by the pressure loss per linear cm toachieve a certain δ. On such basis the tortuous path spacers, referredto above, are quite inefficient. This is apparently due to therelatively large pressure losses at the trailing edge of the rectangularbarriers, the turbulence resulting therefrom being rapidly damped outbefore the next barrier in the flow path is reached. The non-wovenscreens referred to above (oriented to cause frequent 90° changes inflow direction) are much more efficient in terms of it/F achieved for agiven pressure loss per cm flow path length. Most efficient seem to bespacers without any barriers operating in the turbulent flow regime forwhich δ of 0.0005 cm seems readily achievable. δ is apparentlyinsensitive to temperature.

Additional inspection of equation (2) shows that it/F increases as C_(m)(the concentration at the membrane-solution interface) decreases. C_(m)can, in principle, never be less than zero. When C_(m) is so small thatit can be neglected in terms of C for practical purposes, then equation(2) may be written ##EQU6## it/FC is then said to have its "limitingvalue". Equation (3) points out that when C_(m) may be neglected interms of C then it/FC is determined only by D (the diffusion coefficientof sodium chloride), t (the transport number of sodium ions in the CXM),t (the transport number of sodium ions in solution) and δ (the thicknessof the laminar flow layer adjacent to the CXM or half the distance tothe AXM, whichever is smaller). When D, t, t and δ are constant, i (saidin this case to be the "limiting i") is proportional to C (theconcentration of sodium chloride at the mid point between the CXM andthe AXM in the diluting space). It is apparent experimentally that C_(m)never actually becomes zero. The concentration gradient across thelaminar flow layer adjacent to the CXM is constant, i.e. theconcentration of sodium chloride decreases linearly with distance to theCXM through such layer. As a result, when C_(m) may be neglectedcompared to C, there is a (non-linear) electrical potential drop of afew tenths of a volt across the laminar flow layer. The on-set of such apotential drop is most easily seen by plotting E/i versus 1/i (where E/iis the apparent resistance of a cell pair (one diluting space, one CXM,one concentrating space and one AXM)). Such a technique was first usedby Cowan and Brown. A plot of e/i versus 1/i is shown in FIG. 1A. The icorresponding to the inflection point (1) is generally referred to asthe "limiting current density" and may be regarded as the currentdensity at which C_(m) first becomes negligible compared to C. C_(m)actually reaches its steady-state value (close to but not at zero) atinflection point (2). FIG. 1A shows that as i increases (1/i decreases),it reaches a "plateau" at point (1) where it is essentially constantwhile E (and R) increase (E by a few tenths of a volt as mentionedabove). As i increases beyond inflection point (2) (1/i decreases belowsuch point) the current continues to increase but at a voltage levelhigher by a few tenths of a volt. At such latter current, the situationin the laminar flow layer may be regarded as an essentially constantconcentration gradient (C_(m), the concentration at the interfacebetween the CXM and the diluting solution being close to zero) andtherefore resulting in an essentially constant electrical resistance.(The derivative d E/ d i will be essentially constant). The voltagedifference between a 1 normal (1gram equivalent per liter) solution ofhydroxide ion and a 1 normal solution of hydrogen ion at roomtemperature is about 0.8 volts. Therefore as the voltage difference atthe CXM-dilute solution interface approaches a few tenths of a volt,hydrogen ions from the dissociation of water will accompany the sodiumions passing from the diluting solution into the concentrating solution,an equal number of hydroxide ions passing from the interface into thediluting solution. The concentrating solution will then become slightlyacidic and the diluting solution slightly alkaline. However the rate atwhich water can dissociate is relatively small compared to the rate atwhich sodium ions are typically transferred and therefore the changes inacidity and alkalinity respectively are quite small.

The above discussion seems to have covered all that can be learned fromequation (2). However i is the current density locally per cm² ofmembrane area. If the membrane is planar and smooth then the local areais that which may be measured with a straight edge. (It is possiblethat, if a membrane which appears flat and smooth to the "naked eye",were looked at by an electron microscope it would appear rough on ascale of a micrometer or less. However if such roughness is smallcompared to δ, then with respect to equation (2) the membrane is still"flat and smooth"). If the membrane has a surface texture the roughnessof which is comparable with or larger than δ, then the area which mustbe used to calculate i is not the straight edge area but the "actualarea" including such roughness. (Such roughness may also contribute toconverting pressure loss into decreased δ). The "actual area" of a CXMmay be increased also by filling the diluting space between the CXM andthe AXM with cation exchange beads ("CXB"). The increase in area may beeasily calculated from the diameter of such beads. Such filling canincrease the limiting value of it/FC by an order of magnitude or more.(The limiting value of it/FC is that value at which for practicalpurposes, C_(m) (the concentration at the interface between the CXM (andthe CXB)) becomes negligible compared to C).

The above discussion of phenomena taking place at the interface betweenan ion exchange membrane and a dilute solution was confined to CXM. Thephenomena at an AXM are quite similar, with two exceptions. The firstexception arises because the transport number of chloride ions inaqueous solution is 0.6 whereas (necessarily) the transport number ofsodium ions is 0.4. Using equation (2), ##EQU7## where in this case it/Fis the rate of transfer of chloride ions through an AXM, D (as before)is the diffusion constant of sodium chloride in aqueous solution, t is0.6, the following table may be constructed:

    ______________________________________                                                        Relative  Relative                                                                            i Compared to                                 t       t/(t - t)                                                                             it/F      i     CXM at t = 0.95                               ______________________________________                                        0.95    2.71    1.00      1.00  1.57                                          0.85    3.40    1.25      1.40  2.20                                          0.75    5.00    1.84      2.33  3.67                                          0.65    13.00   4.79      7.00  11.00                                         ______________________________________                                    

The last column shows that, comparing an AXM having t=0.95 with a CXMhaving t=0.95, C, C_(m), D and δ being the same, the current density atthe AXM is 57% higher than in the case of a CXM. This means that thecurrent density at an AXM can, in this case, be 57% higher before C_(m)becomes negligible for practical purposes compared to C_(m). Column 3(Relative it/F) shows that utilizing an AXM having t of 0.75 increasesthe relative transport of chloride ions (compared to t=0.95) by a factorof 1.84 (i.e. by 84%) whereas utilizing a CXM having t of 0.75 (comparedto a CXM having t=0.95) resulted in an increase of sodium ion transportof only 24%. (Comparing an AXM having t=0.75 with a CXM having t=0.75,the transport of chloride ion compared to sodium ion at the same C,C_(m) and δ is 57% greater).

The second exception of AXM compared to CXM arises as the voltagedifference at the AXM-dilute solution interface approaches a few tenthsof a volt. In this case hydroxide ions from the dissociation of wateraccompany the chloride ions passing from the diluting solution into theconcentrating solution, an equal number of hydrogen ions passing fromthe interface into the diluting solution. The concentrate solutionbecomes alkaline and the dilute solution acidic. (The changes inalkalinity and acidity as measured by pH changes can be masked by thepresence of carbon dioxide and/or bicarbonate which are pH changebuffers). As the electric current is increased, in this case, the rateat which water dissociates becomes relatively large compared to the rateat which chloride ions are transferred and in fact most of the increasein current is carried by hydroxide ions passing into the concentratesolution. Therefore the changes in alkalinity and acidity can be quitelarge. The increase in alkalinity in the concentrate stream can resultin precipitation of calcium carbonate on the concentrate side of the AXMif (as is frequently the case in "real" aqueous solutions) calciumbicarbonate is present. It is not that somehow the inherent rate ofwater dissociation has increased at an AXM but rather that AXM containand/or have absorbed on their surfaces substances which catalyze thedissociation of water at the high voltage drops which exist at theAXM-dilute solution interface.

Examples are: ##STR1## In equations (4) the ##STR2## (benzyl dimethylamine) groups result from the decomposition of ##STR3## (benzyltrimethyl ammonium) groups in the surface of the AXM. --COO⁻(carboxylate groups) are present, for example, in the tannin likesubstances (generally called "humic" and "fulvic" acids) found innatural surface waters. Such tannin like substances are negativelycharged and are transported (just like other anions) by the electriccurrent to the AXM where they are strongly adsorbed, in part by multipleelectrostatic attractions between positively charged groups ##STR4## inthe membrane and negatively charged groups in the tannin like substance(--COO⁻). Colloidal metal oxides and hydroxides (e.g. iron hydroxide)present in water are also negatively charged and behave as weak acids.It is found that AXM, containing no amine groups (other than quaternaryammonium groups) and processing water free from tannin like substancesand colloidal metal oxides and hydroxides, dissociate water inessentially the same amounts as CXM. Commercial CXM contain sulfonicacid groups (--SO₂ H) which are strongly dissociated so that reaction(5a) does not take place. Most aqueous solutions do not containpositively charged organic or inorganic colloids so reactions similar toreaction (4) above do not generally occur from absorbed substances. Itis possible however to make CXM which do catalyze water dissociation byincorporating in the CXM weakly basic and/or weakly acid groups. Whetherin AXM or CXM, the most effective catalytic groups have ionizationconstants which are equal to the square root of the ionization constantof water (10⁻¹), that is to 10⁻⁷. This may be seen by noting that 10⁻⁷makes the rates of reactions (4b) and (4c) equal. It is possible to makeAXM which do not have weakly basic (or weakly acid) groups and in whichthe bound positively charged anion exchange groups do not decompose intoweakly basic (or weakly acid) groups. Such membranes, when processingclean water, will dissociate water only to the same extent as commercialsulfonate type CXM. Negatively charged organic and inorganic colloidsmay be removed by pretreatment of the solution with ultrafiltration orsalt regenerated anion exchange with highly porous anion exchange resingranules.

The "actual area" of an AXM may be increased by filling the dilutingspace between the AXM and the adjacent CXM with anion exchange resinbeads ("AXB"). Such filling can increase the limiting value of it/FC byan order of magnitude or more. (The limiting value of it/FC is thatvalue at which for practical purposes C_(m), the concentration at theinterface between the solution and the AXM (and the AXB), becomesnegligible compared to C). The diluting space may be filled with AXB bypumping a dilute slurry of AXB into such diluting space or by fillingthe space with AXB before assembling the cell pair. (A "cell pair"consists of one AXM, one diluting space, one CXM and one concentratingspace). Since water dissociation is most important at AXM (and AXB)there is merit in filling the diluting space solely with AXB. If it isdesired to utilize both AXB and CXB granular resins in the dilutingspace then it is important to have as many AXB paths back to the AXM andCXB paths back to the CXM respectively, as possible. When the dilutingspace is filled before assembly of the cell pair such maximization ofpaths may be accomplished by spreading a layer of AXB on the AXM andthen a layer of CXB on the AXB (or, of course, CXB on the CXM and AXB onthe CXB). When the diluting space is filled by pumping, suchmaximization of paths may be accomplished by alternating dilute slurriesof AXB and CXB, the resulting filled space then consisting ofalternating layers of AXB and CXB (in the direction of fluid flow), eachlayer perhaps only a few beads thick. Alternatively the space betweenthe AXM and CXM in the diluting space may be divided by a highly porousscreen, expanded plastic sheet or diaphragm and AXB pumped in on the AXMside and CXB on the CXM side. Even with such filling of the dilutingspace with ion exchange resin beads, it is still possible to arrive atcurrent densities at which the concentration of salt at themembrane-dilute solution and bead-dilute solution interfaces approacheszero. If the current density is increased still further water will bedissociated at the solution-AXB and solution-AXM interfaces intohydroxide ions and hydrogen ions as discussed above. The hydrogen ionswill enter the CXB and CXM along with other cations. Another, closelyrelated, mechanism for water dissociation also exists which may bevisualized by imagining a diluting space of zero thickness, i.e. thedistance between the AXM and the CXM is zero. This is equivalent to adiluting space which is essentially instantly deionized and C_(m) at theAXM-CXM interface approaches zero essentially instantly. When thevoltage drop at such interface reaches a few tenths of a volt water willbe dissociated at the AXM surface into hydroxide ions and hydrogen ions,the former ions passing through the AXM and the hydrogen ions throughthe CXM. (Such a zero thickness diluting space is called a "bipolarjunction" and the pair of AXM and CXM with zero gap is called a"bipolar" membrane. Commercial bipolar membranes contain at theinterface between the AXM and the CXM a thin layer of colloidal metaloxides and/or polymeric organic weak bases and/or weak acids to catalyzewater splitting). A similar situation exists whenever there is aninterface between AXM or AXB and CXM or CXB in which the AXM or AXB isgenerally on the anode (positive electrode) side of such interface(junction) and the voltage drop across such interface is several tenthsof a volt. (If the AXM or AXB is generally on the cathode (negativeelectrode) side of such junction, then the junction acts like a zero gapconcentrating space and very concentrated salt solution (e.g. 15%) willaccumulate at the junction). There seem to have been no experiments todetermine which water dissociation mechanism predominates at currentdensities much above the limiting it/FC, that is, which predominates:water dissociation (a) at AXM and/or AXB-solution interfaces or (b) atAXM and/or AXB junctions with CXM and/or CXB.

In commercial ED apparatus in which the diluting spaces are filled withIX resin beads, such beads seem always to have been a random mixture ofequal gram-equivalents of AXB and CXB, that is about 60 parts of AXB to40 parts CXB, apparently on the impression that such filled dilutingspaces are an electrically regenerated mixed bed ion exchange deionizer.It is true that when the current density is far above that correspondingto the limiting it/CF (so that most of the electric current is carriedby hydroxide and hydrogen ions) then the AXB and CXB will be largely inthe hydroxide and hydrogen ion forms respectively. This may be proved byturning off the electric current whereupon solution passing through thefilled diluting space will continue to be deionized (if the solution isdilute, then for some hours). However it is clear that when the dilutespace is filled with a random mixture of IXB the objective is tomaximize the number of AXB which are connected to the AXM andsimultaneously the number of CXB which are connected to the CXM. If theAXB and the CXB have the same size such maxima clearly occurs (in arandom mixture) when the volume (or number) ratio is 1:1. (Preferablythe conductivities of the AXB and CXB will be the same under actual useconditions. The conductivities depend in known ways on the ionic form ofthe IXB, the water content and ion exchange capacity. Since the ionicform of the IXB may vary substantially from the entrance of a filleddilute space to the exit of such space, it may be preferred to varycontinuously or step-wise the water content and IX capacity of the IXBfrom the entrance to the exit. It is also advantageous if one or bothIXB are short-diffusion-path IXB ("SDP" or shell-and-core IXB), i.e. IXBin which the outer regions contain a normal concentration of groups andthe inner regions contain a much lower (including zero) concentration).If the diameter of the beads is the same as the distance between the AXMand the CXM then all the beads are in contact with both membranes. Forother ratios of bead diameter to AXM-CXM gap and for given ratios ofnumber of AXB to CXB a computer program can be devised to calculate thenumber of AXB which are connected (through other AXB) to the AXM, thenumber of CXB connected to the CXM and the (harmonic) mean length of theAXB and CXB paths. It may be possible to study such problemexperimentally, for example by using a mixture of AXB and inert beads ofthe same diameter between a single pair of AXM, measuring the electricalresistance as a function of membrane gap and the ratio of the number ofbeads of each type. (It may also be possible to study the problem byusing a mixture of copper beads and insulating glass or ceramic beadsbetween a pair of copper plates).

It is generally accepted that whatever IXB are used, the diametersshould all be the same. It is also clear that the surface area of IXBper cm³ of beads is inversely proportional to the diameter of the beads.The pressure drop per unit flow path length obviously also dependsinversely upon bead diameter. IXB used for water softening or chemicallyregenerated deionization typically have diameters of about 0.05 cm.(Such typical bead size is obviously a compromise among practicalpressure loss in an IX bed; duration of exhaustion run; and limitingexchange rate (i.e. avoiding control of such rate by diffusion in theIXB). Filled cell ED clearly involves a different compromise). Althoughsuch diameters (0.05 cm) have also been typically used in ED havingdiluting spaces filled with IXB, it is not clear that such diameters areoptimum for ED. Very much smaller diameters are used for chromatographicanalysis.

IXB are available in so called "gel" types in which the beads aretransparent (but generally colored) and in "macroporous"("macroreticular") types in which the beads are opaque. The gel typeshave water and IX groups more or less uniformly distributed throughoutthe bead. The (tortuous) pores typically have diameters of about 0.002micrometer, varying however with the water content. The macroporoustypes have comparatively large pores (e.g. 0.1 to 1 micrometer), the IXresin with its IX groups forming the walls of such pores. Diffusion inthe large pores is very rapid. The water content of the resin of thepore walls is not generally reported though in principle it ismeasurable. The water content of the gel type IXB generally varies byvarying the amount of crosslinking monomer (for example divinyl benzene)used during suspension polymerization of the beads (for example from amixture of styrene and divinyl benzene). The polymerized beads aretreated with sulfuric acid to introduce sulfonic acid groups therebyproducing CXB or with chloromethyl methyl ether and subsequently with atertiary amine (e.g. trimethyl amine or hydroxyethyl dimethyl amine(also called dimethyl ethanol amine)) to introduce quaternary ammoniumgroups (e.g. ##STR5## Beads containing such IX groups swell whenimmersed in water, the lower the amount of crosslinking and the higherthe concentration of IX groups the greater the amount of swelling. Themost widely used gel type CXB are made from a mixture of styrene and 8%divinyl benzene (100% basis) although such CXB are also availablecommercially with 1 to 12% or more DVB. It has been shown that onlyabout half the DVB in such beads is actually involved in crosslinking.However during polymerization as described above, the growing polymerchains become heavily entangled with each other, such entanglement alsoreducing swelling in water. The most widely used gel type AXB are madefrom a mixture of styrene and 6% divinyl benzene (100% basis) althoughsuch AXB are also available commercially with less or with more DVB. Thesame comments with respect to efficiency of use of DVB and entanglementapply also to AXB. In addition the treatment with chloromethyl methylether (chloromethylation) introduces some methylene crosslinks dependingupon the treatment conditions e.g. the catalyst used. (It is alsopossible to introduce sulfone crosslinks in CXB, depending upon themethod of sulfonation used). It is also possible to make gel type IXB bydiluting the styrene and divinyl benzene with a non-polymerizablesolvent such as diethyl benzene and using comparatively larger amountsof DVB. In such case the swelling of the IXB in water is determined bysuch solvent, the volume of water absorbed being essentially equal tothe volume of solvent used. The efficiency of use of DVB is also roughlyonly 50% but the amount of polymer entanglement is not important.

In dilute solutions IXB generally prefer doubly charged ions (such asCa⁺⁺ and Mg⁺⁺ or SO4⁼) to singly charged ions (such as Na⁺ respectivelyCl⁻ or NO₃ ⁻) and triply charged ions (such as Sc⁺³ (scandium) orFe(CN)₆ ⁻³ (ferricyanide anion)) to doubly charged ions. There is muchconfusion between "doubly charged" and "divalent" and between "triplycharged" and "trivalent". Amphoteric metal hydroxides (that is metalhydroxides which can behave either as weak acids or weak bases such ascupric hydroxide, nickelous hydroxide, ferrous hydroxide, nickelichydroxide, ferric hydroxide, aluminum hydroxide) have the same charge astheir valence only in acid solutions (e.g. +2 for cupric, nickelous,ferrous, +3 for nickelic, ferric and aluminum). The alkaline earthcations (Mg⁺², Ca⁺², Sr⁺², Ba⁺²) are divalent and doubly charged inessentially neutral solutions and the rare earth cations (including Sc⁺³(scandium cation) and La⁺³ (lanthanum cation) are trivalent and triplycharged in essentially neutral solution. The preference for multiplycharged ions compared to singly charged ions may be illustrated by thefollowing reaction:

(6) Ca_(s) ⁺⁺ +2Na_(r) +→Ca⁺⁺ r+2Na_(s) + where the subscript "s" refersto solution and the subscript "r" refers to the CXB resin phase. One canwrite an equilibrium constant: ##EQU8## where the quantities inparentheses are expressed in gram-moles per kilogram of water (or, whatis the same, milligram-moles per gram of water). The equation may berearranged to: ##EQU9## where ##EQU10## is defined as "Y" (not aconstant) for ease in manipulation. The ion exchange capacity of the CXBin gram-equivalents per kilogram water (i.e. the molality) is given by:

    Q=2(Ca.sub.r.sup.++)+(Na.sub.r.sup.+)                      (9)

Then ##EQU11## This is a quadratic equation which has the solution:##EQU12## For example if Y is 250 (liters/mole), corresponding forexample to K=1, Ca_(s) ⁺⁺ =40 ppm and Na_(s) ⁺ =46 ppm, then thefollowing table may be constructed:

    ______________________________________                                        Q             2 Ca.sub.r.sup.++ /Q                                                                   % H.sub.2 O                                            ______________________________________                                        0.1           0.87     97                                                     1.0           0.96     78                                                     10            0.99     26                                                     ______________________________________                                    

The second column (2Ca_(r) ⁺⁺ /Q) is the fraction of the CXB which is inthe Ca⁺⁺ form, the remainder being in the Na⁺ form. The third column (%H₂ O) is the percent by weight of water in a CXB having a dry weightcapacity of 3.5 milligram equivalents per gram of dry CXB, a typicalvalue. The effect of Y ##EQU13## may be seen by setting Y=25 1/mol(consistent with K=1, Ca_(s) ⁺⁺ =400 ppm, (Na⁺)_(s) =460 ppm) from whichthe following table may be constructed:

    ______________________________________                                        Q             2 Ca.sub.r.sup.++ /Q                                                                   % H.sub.2 O                                            ______________________________________                                        0.1           0.64     97                                                     1.0           0.87     78                                                     10            0.96     26                                                     ______________________________________                                    

Although the ratio of Ca⁺⁺ to Na⁺ in solution in the latter table is thesame as in the former table, this total concentration in solution is 10times greater in the latter table resulting in loss of preference forCa⁺⁺ compared to Na⁺ in the CXB. K for Ca⁺⁺ and Na⁺ is not exactly 1 andin addition K is not exactly constant. Nevertheless the above simplecalculation illustrates the principles involved. The conductivity perion (equivalent conductivity) in the case of the CXB having only 26%water is very much less than that having 97% water (the latter having anion conductivity essentially the same as that of water) because offriction between the ions and the polymer of the CXB in the former case(low water content) and the necessity of ions to follow a tortuous paththrough such resin. The ratio of the conductivity of Ca⁺⁺ ion to Na⁺ ionis less in the CXB having 26% water than in that having 97% water, inpart because of the electrostatic attraction of the doubly charged ionfor the fixed sulfonic acid groups. Nevertheless, in ED without fillingin the diluting spaces or with filling in which the CXB portion isprepared from styrene-divinyl-benzene mixtures having 8% DVB, Ca⁺⁺ isgenerally preferentially removed compared to Na⁺, at least at currentdensities such that C_(m) (the total concentration at the CXM and/or CXBsolution interface) is not far removed from C (the concentration outsidethe laminar flow layer adjacent to the CXM or CXB).

Similar considerations apply to the comparative absorption andconductivity of doubly charged sulfate anions compared to singly chargedchloride anions or nitrate anions in AXM or AXB. (K for nitrate ionscompared to chloride ions is about 2 for most AXM or AXB). However itappears that bulky quaternary ammonium groups such as ##STR6## excludesulfate compared to chloride (or nitrate) as compared with the commoncommercial quaternary group ##STR7## Such exclusion appears to be due inpart to the lack of ability of the doubly charged sulfate ion toapproach closely the positively charged nitrogen atom.

The electrodialytic performance of IXM and IXB is however not determinedsolely by the equilibrium absorption of ions by the IXM or IXB or by therelative conductivities of the ions in the membranes or beads. It ispossible to make thin skins on the surfaces of IXM and IXB which skinsretard the passage of doubly and triply charged ions. (The skins havelow ion exchange capacities, low water contents and low dielectricconstants). Such membranes are called "univalent ion selectivemembranes" and are typically used to prepare 18 to 20% sodium chloridebrine by ED from sea water, permitting the passage of sodium andchloride ions and inhibiting the passage of magnesium, calcium andsulfate ions. (The use of the term "univalent ion selective" is notaccurate; it would be better to say "selective for singly charged ions"but since the membranes are almost always used to separate ions whichhave the same charge as their valency (but what is the "valency" ofsulfate?) there is no practical harm in the former term). In the aboveconcentration of seawater by ED using such skinned membranes, thecurrent density used is far less (e.g. 50% or less) than thatcorresponding to the "i" in the limiting value of it/FC (that is to the"i" at which C_(m) is negligible compared to C, the--bulk concentrationin the diluting space). At or above the limiting it/FC what goes througha membrane is essentially what is presented to the membrane by diffusionand conduction in the laminar flow layer adjacent to the membrane. Theratio of the ions arriving at the membrane is the ratio of the limitingit/F that is the ratio ##EQU14## Univalent anion selective membraneshave been used selectively to remove nitrate from water containing alsosulfate and/or bicarbonate or (with CXM which are not univalent ionselective) selectively to remove calcium chloride from water containingsodium, calcium, chloride, sulfate and bicarbonate ions. In each casethe it/FC actually used is 50% or less than the limiting it/FC.

It was noted above that the ratio of the conductivity of doubly chargedions compared to singly charged ions is less in IXB having low watercontents than in IXB having high water contents, in part because of theelectrostatic attraction of the doubly charged ions for the fixed(singly charged) ion exchange groups. It was also pointed out thatbulky, quaternary ammonium anion exchange groups tend to exclude doublycharged sulfate ions in part due to the inability of the doubly chargedions to approach closely the positively charged nitrogen atom in thequaternary ammonium exchange group. One might therefore expect that suchdiminished electrostatic attraction would lead to greater conductanceper sulfate ion at the same water content and same AXB molality(gram-equivalents of fixed, anion exchange groups per kilogram of waterin the AXB). A recent report se to confirm that sulfate is more mobilein an AXB having ##STR8## AX groups than in an AXB having ##STR9##groups. Sulfate mobility may be even higher in AXB having ##STR10##(benzyl tributyl ammonium) groups. However, as noted above, the lattergroups highly exclude sulfate and therefore the relative transport ofsulfate versus chloride from solutions containing both ions (at it/FCsubstantially less than the limiting it/FC) may be poor even though thesulfate ion mobility is comparatively high. It would appear that"pleasingly plump" (rather than "bulky") groups are indicated. Inaddition to (CH₃)₂ N⁺ CH₂ CH₂ OH groups, these may include:

(CH₃)₂ N⁺ CH₂ CH₃

CH₃ N⁺ (CH₂ CH₃)₂

N⁺ (CH₂ CH₃)₃

CH₃ N⁺ (CH₂ CH₂ OH)₂ and

N⁺ (CH₂ CH₂ OH)₃.

Commercial AXB are available having N⁺ CH₂ CH₃)₃ groups. Such AXB appearto be macroporous and it is not known whether sulfate mobility will bethe same in a macroporous AXB having such groups as compared to a geltype AXB with such groups. All commercial ED apparatus having IXB in thediluting spaces use a mixture of AXB and CXB. All such apparatus use asCXB gel type beads having ##STR11## as the exchange groups, known forexample as Dowex 50 (Dow Chemical Co.) or Amberlite IR 120 (Rohm andHaas Co.). Most such apparatus use as AXB gel type beads having##STR12## exchange groups (known as Type I groups) for example Dowex 1or Amberlite IRA 400. The remaining commercial apparatus used (at leastoriginally) AXB gel type beads having ##STR13## exchange groups (knownas Type II groups) for example Dowex 2 or Amberlite IRA 410. In the1960's, an in-house apparatus used a macroporous (macroreticular) AXBhaving Type I AX groups (Dowex 21K was actually used; similar AXB areavailable from other manufacturers) to supply deionized water forlaboratory use.

(Macroporous (macroreticular) IXB are made, for example, from a mixtureof styrene and divinyl benzene with a diluent which is a solvent for thestyrene and DVB and a poor solvent for the polymer of styrene and DVB.As a result, as polymerization proceeds the polymer precipitates fromsolution leaving macropores. The size of the macropores depends upon thequantity of the diluent and its solubility for the polymer. The amountof DVB used may be the same as used in gel type IXB or less because thepolymer in macroporous IXB tends to be highly entangled, the extentdepending on the diluent used. With some polymeric diluents it ispossible to use no DVB).

Almost all of the above discussion has pertained to the diluting spacein an ED apparatus. The design of the concentrating space cannot,however, be neglected. The concentrating space should contain structureto keep the adjacent IXM flat and against whatever structure (turbulencepromoting spacer or IXB) may be present in the adjacent diluting spaces.The structure in the concentrating space should also assist the adjacentIXM to resist whatever hydraulic pressure difference there may be fromthe diluting side of the membranes to the concentrating side. (It iscommon to operate the diluting space at a higher pressure than theconcentrating space in order to avoid possible leaks of concentratesolution into the dilute solution). The concentrating space may beidentical in structure to the diluting space e.g. have the samethickness, the same turbulence promoting structure or the same IXB. Insuch case the choice of which electrode of the pair (discussed above) ispositive and which is negative is arbitrary. If the solution processedin the diluting space contains organic or inorganic colloidal matterand/or poorly soluble salts, then the colloidal matter can accumulate atone or the other of the diluting surfaces of the IXM and/or IXB and thepoorly soluble salts at one or the other of the concentrating surfacesof the IXM and/or IXB. Regular, periodic reversal of the polarity of theelectrodes will then convert a diluting space into a concentrating spaceand a concentrating space into a diluting space, generally effectivelyremoving accumulated colloids and poorly soluble salts. Cycle times of30 minutes to 2 weeks are used commercially depending upon the severityof accumulation of colloids and poorly soluble salts (that is reversingpolarity every 15 minutes to one week). It is not necessary that suchreversal be symmetric (that is, that each space spend equal time as adiluting space and a concentrating space). It is equally possible tooperate very asymmetrically say 15 minutes in one direction and 15seconds in the other. In the case of substantially symmetric reversalwhatever structure is optimal for diluting and concentrating spaces inone direction is obviously also optimal in the opposite direction andtherefore the structure (e.g. non-woven screen or IXB) should be thesame in both spaces. In the case of asymmetric or no reversal thestructure in the two types of spaces need not be the same. For examplethe distance between the AXM and the CXM in the concentrating space canbe very much less than such distance in the diluting space and such thinconcentrating space can have structure enabling a single pass ofconcentrate solution through the concentrating space without recycle(nevertheless at a pressure loss which is only slightly less than thatin the diluting space) thereby avoiding the investment and operatingcost of concentrate solution recycle. A single pump can be used to feedboth concentrating and diluting spaces while still maintaining thedesired ratio of effluent flows (i.e. percent of feed which is recoveredas deionized product). Instead of (or in addition to) usingconcentrating spaces which are thinner than the diluting spaces (toeliminate the need for recycle of concentrate) it is possible to fillthe concentrate space with IXB having a smaller mesh size (smallerdiameter) than the IXB used in the diluting spaces, the smaller diameterresulting in greater hydraulic resistance. Of course attention must bepaid to achieving the necessary δ in the concentrating space to assureadequate mass transfer of poorly soluble electrolytes (e.g. silicic acid(silica), calcium bicarbonate and calcium sulfate) from the interfacesbetween the IXM (and IXB, if such are used in the concentrating space)and the concentrating solution into the bulk of such solution.

If the diluting solution being deionized is already very dilute (e.g. 6ppm as sodium chloride) then, even at 90% recovery of deionized dilutesolution the conductivity of the concentrating solution effluent will beonly about 100 micro Siemens/cm. If in such case IXB filling is used inthe diluting space (which will be highly desirable to increase theeffective current density in the diluting space and to decrease theelectrical resistance of such space), then the principal electricalresistance of the cell pair will be in the concentrating space and itwill be desirable to use IXB filling also in the latter space. Such IXBfilling need not be the same as the filling in the diluting space. Forexample in the above mentioned filled cell ED apparatus used in-house inthe 1960's, it was found advantageous to fill the concentrating spacessolely with weak base AXB.

The above discussion has concentrated on the use of IXB as filling inthe diluting and/or concentrating spaces. The literature also reportsapparatus in which the surface of AXM was corrugated at an angle of 45°to the direction of flow, the CXM corrugated at -45° and thecorrugations of the AXM were in contact with those of the CXM. The flowwas thereby forced to make frequent 90° changes in direction. Apparatusis also reported in which AX fabric was placed against the AXM in thediluting space and CX fabric against the CXM in the same space. Thefabrics were knit from IX fibers. CX fibers are most easily made frompolyethylene mono- or multi-filaments by soaking the latter in a mixtureof styrene, divinyl benzene and free radical catalyst, polymerizing themonomers and subsequently sulfonating. AX fibers may be made bysubstituting vinyl benzyl chloride for the styrene and finally treatingthe filaments with an appropriate tertiary amine as discussed above. (IXfibers and filaments may also be made by sulfochlorinating polyethylenefibers and filaments, subsequently hydrolyzing or aminating andquaternizing). Such IX monofilaments may be easily bonded together bymoderate heat and pressure to give non-woven screens. The latter canobviously be made on automatic machinery. The literature reports thatthe highest values of limiting it/FC were obtained when there was someinterpenetration of the AX knitted fabric with the CX knitted fabric. IXmonofilaments or multifilaments having a diameter equal to the spacingbetween the AXM and the CXM may also be arranged in alternation parallelto the direction of flow.

In the above discussion limiting values of it/FC were used as the basisof discussion since such ratio reflects the actual transfer of ions##EQU15## In engineering practice it is common to use the ratio##EQU16## because it relates the easily measurable i and C. It does nothowever relate to the performance of the ED apparatus.

PROBLEMS ADDRESSED IN THIS APPLICATION

In commercial, serial production, ED stacks (whether or not with IXBfilling in at least the diluting spaces) cost about 25% of the totalassembled equipment cost in the case of small ED plants and about 50% inthe case of large plants. Such stacks may consist of a thousand or morecomponents (not including the IXB), each of which may require severalmanufacturing steps. IXM are generally manufactured by one of thefollowing processes:

(a) A fabric woven from synthetic staple fiber is impregnated with amixture of sulfoethyl methacrylate, ethylene glycol dimethacrylate withor without divinyl benzene, a free radical catalyst and anon-polymerizable diluent. The impregnated fabric is heated to causepolymerization of the monomers. The resulting sheet is leached withdilute sodium bicarbonate solution to remove the diluent. The diluent isa good solvent for both the monomers and the polymer and the latter istherefore gel-like. The manufacturing process is essentially continuous.

(b) A fabric woven from synthetic staple fiber is impregnated with amixture of ##STR14## ethylene glycol dimethacrylate, a free radicalcatalyst and a non-polymerizable diluent. The impregnated fabric isheated to cause polymerization of the monomers. The resulting sheet isleached with water to remove the diluent. The diluent is a good solventfor both the monomers and the polymer and the latter is thereforegel-like. The manufacturing process is essentially continuous. The AXMmade by this process are not stable in caustic.

(c) A fabric woven from synthetic monofilament is impregnated with amixture of styrene, divinyl benzene, polystyrene and a free radicalcatalyst. The impregnated fabric is heated to cause polymerization ofthe monomers. The resulting sheets are treated with sulfuric acid tointroduce sulfonic acid groups thereby producing CXM or withchloromethyl ether and subsequently trimethyl amine to produce AXM. Themanufacturing process is essentially continuous.

(d) A fabric woven from synthetic monofilament is impregnated with astyrene-butadiene latex and allowed to dry. The impregnated fabric istreated with sulfuric acid to crosslink the polymer and introducesulfonic acid groups thereby producing CXM or with the chlorides ofmetals which form weakly acidic hydroxides (such as titanium chloride)to crosslink the polymer, then with chloromethyl ether and subsequentlywith trimethyl amine to produce AXM.

(e) A paste is prepared from styrene, divinylbenzene, dimethyl phthalate(a non-polymerizable diluent), a free radical catalyst and powdered PVC.The paste is calendered into a fabric woven from monofilament. The thusimpregnated fabric is heated to cause polymerization of the monomers.The resulting sheet is treated with sulfuric acid to introduce sulfonicacid groups thereby producing CXM or with chloromethyl ether andsubsequently trimethyl amine to produce AXM. The manufacturing processis essentially continuous.

(f) About 25 parts of polyethylene are masticated on a rubber mill andabout 75 parts of IX powder are added. The mixture is calendered intoand on a fabric woven from monofilament. Preferably the IX powderparticles are spherical. If the IX powder is AX powder the final productis an AXM. If the IX powder particles are CXB the final product is aCXM. The manufacturing process is essentially continuous.

A typical separator between the IXM in an ED stack uses a combined frameand spacer having a tortuous flowpath. Polyethylene is extrudedcontinuously as a sheet 0.05 cm thick. The resulting continuous sheet iscut with a rotary die to form 6 or 8 flowpaths having rectangularbarriers perpendicular to the flowpath about every 2.8 cm. The patternof the flowpath is asymmetric so that when one die cut sheet is turnedover and placed on another, the barriers occur about every 1.4 cm in theflowpath, alternating from the AXM side of the flowpath to the CXM side.The two thus oriented die cut sheets are glued together manually to forma separator 0.1 cm thick. These separators are inexpensive butinefficient.

Another common separator between the IXM in an ED stack consists of aframe, for example of polyethylene, ethylene vinyl acetate copolymer,ethylene-ethyl acrylate or ethylene-acrylic acid copolymer, and anon-woven or expanded plastic screen of the same thickness in the spaceenclosed by the frame. The screen may simply lie on the membraneadjacent to the frame during assembly of the ED stack or the screen maybe imbedded in the frame. The literature discloses many designs ofscreen type separators and many methods of making such. Screen typeseparators can be efficient. It is necessary to assure that the flow ofsolution through the screen type separators is uniform. At the same timeit is necessary to avoid cross-leaks between concentrating and dilutingspaces and to avoid by-passing of the applied direct current down theinternal manifolds, particularly the concentrate solution manifold. AnED stack may be regarded as a network of electrical resistances e.g.

one path consisting of all the AXM, dilute spaces, CXM and concentratespaces in series;

paths from each concentrating space down the entrance and exit channelsof each such space, through the concentrate manifolds to the entranceand exit channels of all the other concentrating spaces and into allsuch spaces;

paths from each diluting space down the entrance and exit channels ofeach such space, through the dilute manifolds to the entrance and exitchannels of all the other diluting spaces and into all such spaces. (Theeffluent from the diluting spaces is much more resistant than theeffluent from the concentrating spaces and therefore the electriccurrent lost through the dilute effluent manifold is much less than thatlost through the concentrate effluent manifold. The influent to thediluting spaces may or may not be more resistant than the influent tothe concentrating spaces. Therefore the current lost through the diluteinfluent manifold may or may not be less than that lost through theconcentrate influent manifold);

paths laterally through each IXM to any dilute or concentrate manifoldwith which such IXM may be in contact.

Although such network of electrical resistances consists of hundreds ofresistances in series and parallel, there are only a few elementalresistances each of which is repeated in a definite pattern in thenetwork. Such networks have therefore been solved for the electricalcurrent in each branch. For the sake of efficiency of the apparatus, itis generally desirable to limit the average by-pass current loss percell pair to about 1% of the total current per cell pair. Excessivelateral currents in IXM under the spacer frames can result in serioustemperature increases in such region and distortion of frames and/ormembranes.

Thus the separators between IXM must:

assure equal distribution of dilute and concentrate solutions amongseveral hundred diluting and concentrating spaces respectively;

assure uniform velocities in each diluting space and in eachconcentrating space;

limit leakage of solutions to the outside of the ED stack;

limit cross-leak between the dilute and concentrating solutions;

limit by-pass electrical currents. To achieve all of the above, somespacer-frames consist of:

separate frames containing holes for the internal manifolds;

separate spacers for positioning and supporting the IXM and forpromoting turbulence in the flowing solutions to reduce δ, such spacersbeing inserted into the frames by hand during assembly of the ED stack;

separate inserts (for distributing solutions evenly among the respectivespaces, for limiting cross-leaks and by-pass currents) inserted into theframes by hand during assembly of the ED stack.

In other spacer-frames the above functions are integrated into amonolithic structure, nevertheless often requiring several machineand/or hand operations for manufacture of each spacer frame.

As noted above, diluting and/or concentrating spaces may be filled withIXB (a) by spreading such IXB by hand or machine on one or the other IXMand within an appropriate frame or (b) by pumping such IXB into anassembled ED stack by fluidizing the IXB. Method (a) permits AXB to belayered against the AXM and CXB against the CXM, a junction between theAXB and CXB occurring somewhere between the AXM and CXM. The location ofsuch junction may be adjusted in accordance with the characteristics ofthe AXB and the CXB and the composition of the solution being treated.U.S. Pat. No. 2,923,674 points out that when the diluting spaces arefilled solely with CXB then, when polarization occurs it will occur atthe junction between the CXB and the AXM, the concentrating solutionwill become alkaline and the diluting solution acidic. Furthermore whenthe diluting spaces are filled solely with AXB then, when polarizationoccurs it will occur at the junction between the AXB and the CXM, theconcentrating solution will become acidic and the diluting solutionalkaline. Such observation was made also by Desalination 16 (1975)225-233 which discloses using CXB near the CXM and AXB near the AXM.Amberlite IRA 402 (Type I AXB having low crosslinking (6%) and 54% wateron a wet basis, available from Rohm & Haas Co., Philadelphia, Pa.) waspreferred in such configuration to IRA 400 (standard crosslinking (8%)and 45% water) because 402 has lower electrical resistance. The aboveDesalination publication notes that "In choosing the type of resin,interaction between bivalent and trivalent ions and the resin must betaken into account. Such interaction can dramatically increase theelectrical resistance of the stack." For example such publicationreported for Amberlite 120 (standard XL (8%), 45% water) and forAmberlite IRA 400 the following relative resistance (vs. Na⁺ and Cl⁻forms resp.):

    ______________________________________                                        Form   Relative Resistance                                                                          Form   Relative Resistance                              ______________________________________                                        Na.sup.+                                                                             1.00           Cl.sup.-                                                                             1.00                                             Ca.sup.++                                                                            4.33           SO.sub.4.sup.=                                                                       1.61                                             Mg.sup.++                                                                            5.42           CO.sub.3.sup.=                                                                       1.28                                             ______________________________________                                    

Method (b) above (pumping IXB into assembled ED stacks) permits the IXBeasily to be layered in the direction of flow through the dilutingand/or concentrating space (see U.S. Pat. Nos. 5,066,375; 5,120,416;5,203,976).

"For example:

a) the size of the particulates AX and CX can be varied along theflowpath in compartment(s) 10 . . . ;

b) the relative blend of AX and CX particulates can be varied along theflowpath. For example it is possible easily to make a multi-layeredpacking e.g. of alternating AX and CX particulates or the lower part ofcompartment 10 can contain such layered packing and the upper partrandomly mixed particulates;

c) the portion of the packing in compartment 10 which is the first tocontact fluid can have special properties; e.g. it can be non-ionexchange particulates or organic scavenging particulates even if suchare poor electrolytic conductors or even electrical insulators."

The above mentioned patents suggest that the ion exchange particulatescan be beads or spheres or "any structures which provide fluidinterstices and permit flow of such fluid in the interstices, forexample irregular granules, thin rods preferably parallel with thesurfaces of the membranes, fibers including woven or knitted fibers,saddles, rings, tellerettes, etc."

The above mentioned patents also disclose that the IXB can be removedfrom an assembled ED stack by reversing the direction of flow throughthe stack. However equipment for hydraulic removal of and refilling withIXB may not be available in the field in which case removal andrefilling may be very inconvenient, sometimes requiring return of filledstacks to the factory for service.

(Some filled cell ED stacks have the spacer-frames sealed to the IXmembranes (see e.g. "Electro-Regeneration of Ion-Exchange Resins" FinalReport [of Southern Research Institute] to Artificial Kidney-ChronicUremia Program of the National Institute of Arthritis and MetabolicDiseases, PB 210,163 (1972) page 55). Such stacks can of course bedesigned to permit filling with and removal of IXB hydraulically but ifnot so designed then the stacks must be returned to the manufacturer forservice. The apparatus disclosed in the above mentioned Final Reportoperated at constant current, used IXB of substantially uniform size(e.g. Rohm and Haas Stratabed-84 and Stratabed-93) as well as Type IIAXB (e.g. Amberlite IRA 410 and 910 and Duolite A-102D).

Thus it is desirable to have more convenient ways of filling ED stackswith IXB and removing such IXB from such stacks while at the same timeachieving any desired geometric arrangement of the IXB. Other objectswill be obvious from the following examples and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation in plan view, not necessarily toscale, of a frame 10 useful in this invention and, in negative, of apattern or mold for preparing such a frame.

FIG. 1A shows a plot of E/i versus 1/i for a laminar flow layer.

FIG. 2 is a schematic representation in plan view, not necessarily toscale, of a frame 20 and pillow(s) (or pocket(s) or packet(s)) useful inthis invention, the pillow(s) containing a body (bodies) of ion exchangepolymer permeable to bulk flow of fluid.

FIG. 3 is a schematic representation in plan view, not necessarily toscale, of a frame 30 and other pillow(s) useful in this invention, thepillow(s) also containing a body (resp. bodies) of ion exchange polymerpermeable to bulk flow of fluid.

FIG. 4 is a schematic representation in side view, not necessarily toscale, of a pillow of FIG. 2.

FIG. 5 is a schematic representation in side view, not necessarily toscale, of a pillow of FIG. 3. FIG. 6 is a schematic representation, notnecessarily to scale of a frame 60 useful in this invention, the framecontaining a multiplicity of pillows.

INVENTIONS ADDRESSED TO SUCH PROBLEMS

Integral, Monolithic Membrane--Frames:

It has been pointed out above that it is desirable to reduce the numberof individual components in an ED stack; to reduce the number ofoperations required to construct a cell pair; and to simplify theassembly of an ED stack. These and other objectives are attained by thefabrication and use of integral, monolithic membrane-frames. U.S. Pat.No. 4,804,451 discloses a filled, dilute ED compartment having a spaceradhered on one surface directly to the reinforcing fabric of an AXM andon the other surface directly to the reinforcing fabric of a CXM. The IXresin of the IXM is skived off, down to the reinforcing fabric, in thearea in which the spacer will be adhered. Adherence is provided bysolvent-based adhesive; by hot-melt adhesive; by direct heat weld; bythermoplastic interlays; by contact adhesive; or by pressure sensitivehot melt adhesive. It is clear that such article (and the method ofmanufacturing it) adds several operations to the assembly of a filledcell ED stack:

the IX resin must be skived off an IXM in a definite pattern, down tothe reinforcing fabric;

adhesive must be applied to the skived areas and/or one side of aspacer;

the spacer must be adhered to the IXM;

the cavity in the spacer must be filled with IXB;

IX resin must be skived off a second IXM in a definite pattern (themirror image of the pattern first mentioned above);

adhesive must be applied to the latter skived areas and/or the secondside of the spacer; and

the second IXM must be adhered to the spacer.

Such a filled, dilute ED compartment is totally sealed (except forentrance and exit channels). It does not consist of an integral,monolithic membrane-frame. Such a filled, sealed, dilute ED compartmentcannot be maintained either in the field or in the factory and must bethrown away for any defect which develops which defect significantlyaffects performance of the filled cell ED stack.

EXAMPLE 1

FIG. 1 is a schematic representation of the plan view, not necessarilyto scale, of a frame 10 suitable for use, according to this invention,in ED (including reversing type ED ("EDR"); filled-cell ED ("EDI");reversing type, filled-cell ED ("EDIR"); ED with bipolar membranes("BPED")); reverse osmosis ("RO"); nanofiltration ("NF");ultrafiltration ("UF"); microfiltration ("MF"); gas separation ("GS");pervaporation ("PV"); diffusion dialysis ("DD"); Donnan dialysis("DnD"); piezodialysis ("PD"); membrane distillation ("MD"); diffusionosmosis ("DO"); thermo osmosis ("TO") and electrolysis ("EL") and othermembrane processes. In the figure, 11a and 11b are cavities in the frame10. The frame may consist of a single cavity or many cavities eventwenty or more. 12a and 12b represent manifold apertures (e.g. entrancemanifold apertures, alternatively exit manifold apertures). 14a and 14balso represent manifold apertures (e.g. exit manifold apertures if 12aand 12b are entrance manifold apertures, otherwise entrance manifoldapertures). 16a, 16b, 17a and 17b represent internal manifold aperturesnot connected with cavities 11a and 11b and forming internal manifoldsfor other fluid streams in the apparatus of which frame 10 is a part.13a, 13b, 15a and 15b represent conduits communicating between theadjacent manifold apertures and the cavities. 18 (repeated several timesin the figure) designates the solid material of the frame e.g. a hardpolymer, a resilient polymer or a laminate of hard polymers withresilient polymers. It will be clear that a pattern or mold for makingframe 10 will be the negative of FIG. 1, e.g. 18 will represent milledout portions of the pattern whereas the other numbers will representareas not milled out.

According to this example, the pattern of the frame represented in FIG.1 is milled into an aluminum plate. The depth of the milled out area (18in FIG. 1) is about 3 mm. The effective area of each flow path is about732 cm² and the flowpath length is about 84 cm. The aluminum plate iscoated with polytetrafluoroethylene. A steel die is prepared having thesame pattern. Acrylic non-woven fabric (Carl Freudenberg T 29516) is cutto the pattern with the steel rule die and then soaked in a mixtureconsisting of (all parts by volume except as indicated):

    ______________________________________                                        Methacryloxyethyl trimethyl ammonium                                                                 39 parts                                               chloride (80% in water) [51410-72-1]                                          Ethylene Glycol Dimethacrylate [97-90-5]                                                             21 parts                                               Dipropylene Glycol     40 parts                                               Azobisdimethyl isobutyrate                                                                           13 grams per liter                                     ______________________________________                                    

The pieces are drained and placed one by one in the milled-out patternuntil the pattern is filled. A piece of the same fabric about 28 by 107cm is soaked in the above mixture and placed over the pattern assuringgood contact everywhere with the top-most cut piece in the pattern. AMylar (TM duPont Co., Wilmington, Del.) sheet about 0.008 cm thick isplaced over the 28×107 cm fabric and covered by a piece of flat glassabout 32×110 cm. The resulting construction is heated in a recirculatingair oven at 85° C. for about 4 hours to effect polymerization. It isremoved from the oven, allowed to cool until it is only warm to thetouch and the flat glass and Mylar film are removed. The patterncontaining the integral, monolithic AXM-frame is immersed in 2N sodiumchloride solution for about 1 hour. The AXM-frame is then removed fromthe pattern and stored in 2N sodium chloride solution. Excess AXMextending beyond the frame is removed.

EXAMPLE 2

The procedure of Example 1 is repeated except:

polyester fabric (Style 66149 from Precision Fabrics Group Inc.)replaces the acrylic fabric; and

the polymerizable mix is (parts by volume except as indicated):

    ______________________________________                                        2-sulfoethyl methacrylate [1804-87-1]                                                                 35 parts                                              ethylene glycol dimethacrylate [97-90-5]                                                              11 parts                                              80% divinyl benzene [1321-74-0]                                                                       11 parts                                              dipropylene glycol      43 parts                                              azobisdimethyl isobutyrate                                                                            20 grams/liter                                        ______________________________________                                    

Monolithic, integral frame-membranes are also made in similar fashion toExamples 1 and 2 having "half-thickness" frames on each side of eachmembrane. It is found that there is less distortion in such "doublesided" frame-membranes when passing from one equilibrating solution toanother. Alternatively monolithic, integral frame-membranes are madehaving full thickness frames on each side of each membrane. It is foundto be advantageous to use CXM having integral full-thickness frames oneach side and standard flat AXM since in actual use the lifetimes of AXMare generally significantly less than those of CXM. Replacement costsare thereby reduced.

EXAMPLE 3

A five cell pair filled cell ED stack is assembled using integralmonolithic framed membranes prepared in accordance with Examples 1 and2. The end blocks are PVC into which has been milled recesses to acceptelectrodes. The electrodes are platinum electroplated titanium sheet. Onthe lower end block there is placed a frame made in accordance withExample 2 but omitting the 28×107 cm fabric. The open space in suchelectrode frame is filled with non-woven screen except that at the endsof the frame flexible urethane foam is placed to block flow from theconduits to and from the internal manifolds. The urethane foam isslightly thicker than the electrode frame. Hydraulic connections to theelectrode frame flowpaths are made through the PVC end blocks andelectrodes. Both concentrating and diluting spaces are filled with IXB.The areas near the entrance and exit channels are filled with non-wovenscreen in order to confine the IXB to the flowpaths. When, during theassembly of the stack, the membrane exposed is an AXM then about 125 mlof Purolite Purofine A-300 are spread uniformly over the exposedmembrane and about 84 ml of Purolite Purofine C-100 EF are spreadcarefully and uniformly over the A-300. When the membrane exposed is aCXM then about 85 ml of C-100 EF are spread uniformly over such membraneand about 125 ml of A-300 carefully and uniformly over the C-100 EF. Ineither case the amount of the second IXB spread is that which completelyfills the space.

(The Purolite IXB may be obtained from the Purolite Co., Bala Cynwyd,Pa. Purofine A-300 is a polystyrene based Type II clear gel type AXBhaving a water content of 40 to 45% by weight on a wet, drained basisand having about 98% of the particles in the size range 0.42 to 0.71 mm.Purofine C-100 EF is a polystyrene based gel type CXB having sulfonicacid exchange moieties, a water content of 46 to 50% by weight on a wet,drained basis and having at least 95% of the particles in the size range0.40 to 0.60 mm).

The stack is tightly clamped together by means of steel end plates andthreaded tie-rods. The stack is used further to demineralize thepermeate from a reverse osmosis apparatus, which permeate has a pHaveraging about 6 and an electrical conductivity averaging about 1micro-Siemen/cm. The flow to each diluting cell is about 10 ml/sec. Thefeed and outlet pressures of the concentrating and electrode streams areadjusted to minimize crossleaks. The feeds and effluents of the variousclasses of compartments are carried through coils of long pieces offlexible tubing in order to reduce by-pass currents. A d.c. current ofabout 0.5 amperes is applied to the stack. It is found that the effluentfrom the diluting compartments has a conductivity averaging about 0.1micro-Siemens/cm, i.e. about a 90% reduction in conductivity.

COMPARATIVE EXAMPLE 1

Frames are made in accordance with Examples 1 and 2 but in each caseomitting the 28×107 cm fabric. The latter fabric is converted to normal,flat AXM and CXM by soaking pieces either in the polymerizable mix ofExample 1 or that of Example 2 respectively. A piece of Mylar film (asdescribed in Example 1) about 34 by 113 cm is carefully laid over apiece of flat glass about 32×111 cm, removing wrinkles in the film. Apiece of fabric which has been soaked as described above is laid overthe film, care being taken to eliminate bubbles and wrinkles. A secondpiece of Mylar film is laid over the fabric followed by a second glassplate, a third Mylar film, a second piece of soaked fabric etc. until aconvenient stack of glass plates, film and impregnated fabric isachieved. The resulting stack is heated as described in Examples 1 and2, cooled and disassembled. The resulting flat membranes are stored in2N sodium chloride solution.

A stack of five cell pairs is assembled as described in Example 3, usinghowever the separate frames and membranes. The membranes are dye testedfor leaks, trimmed to the size of the frames and holes are cut in themto form with the frames the internal manifolds for flow distribution. Itis found that assembly of the filled cell ED stack is much more timeconsuming with the separate frames and membranes than with the integralframe-membranes of Example 3.

The stack is operated as in Example 3. It is found that the minimumcrossleak attainable when the inlet and outlet pressures are carefullyadjusted is larger than that obtained in Example 3. Under the operatingconditions of the latter example, demineralization is found to be lessthan 85%.

EXAMPLE 4

In the case of the stack of Example 3, the monolithic integralframe-membranes result in a series of contacts between alternating AXand CX resin. Whenever AX resin is on the anode side of a junction withCX resin, then such junction is a demineralizing, bipolar junction. Anyby-pass current through the frame portions of the integralframe-membranes will therefore result in rapid depletion of electrolyteat the above mentioned demineralizing junction. Any by-pass current istherefore self-limiting at low cell-pair voltages. However when the cellpair voltage exceeds about 1 volt, sufficient potential is available tosplit water into hydrogen and hydroxide ions. The resulting by-passcurrent does not seriously detract from the feasibility of theapparatus, particularly when both concentrating and diluting cells arefilled with IXB. Nevertheless it is possible to reduce such by-passcurrent. In this example such is accomplished by die-cutting vinylchloride-vinylidene chloride copolymer film to the shape of the frames.The stack of Example 3 is disassembled and reassembled as in Example 3,except at those frame junctions (concentrating junctions) at which CXresin is on the anode side of the junction, a single piece of suchdie-cut film is placed over the composite frame-membrane.

Such operation increases the time of assembly and it is believed that asingle insertion of such film at each end of the stack, or at everythird or fourth concentrating junction, would significantly reduceby-pass current through the frames at high cell-pair voltages.

EXAMPLE 5

By-pass current through IX resin frames of Example 3 can be eliminatedby using separate insulating frames and flat membranes. Such insulatingframes require an additional molding or machining operation and doublethe number of pieces which must be handled as discussed in ComparativeExample 1. The film inserts of example 4, even if used infrequently in astack, increase labor cost. This example discloses an alternateprocedure.

Composite frame-AXM are made as in Example 3. The frame portions of someof such composite structures are dipped in a synthetic rubber latex andallowed to drain. The frame portions of other such composite structuresare painted with such latex with a roller. A stack is assembled andtested as in Example 3 using such coated frame-AX membranes and uncoatedframe-CX membranes. Performance is found to be about the same as inExample 3. The coating operation is not significant compared to theassembly time of a stack. It is believed that a satisfactory reductionof by-pass current will be achieved by coating only a small fraction ofthe AX and/or CX composite frame-membranes.

EXAMPLE 6

Ethylene-vinyl acetate copolymer sheet having a thickness slightlygreater than the depth of the aluminum pattern of Example 1 is cut tothe pattern of FIG. 1 and inserted into the aluminum pattern. YuasaBattery Co. (Tokyo, Japan) MF250B microfiltration membrane sheet havingdimensions of about 28×107 cm is placed over the aluminum pattern (andcopolymer) and covered with a piece of the Mylar film described inExample 1. A steel plate is placed over the film and the resultingstructure heated at 85° C. for four hours. It is found that thecopolymer has penetrated into the adjacent pores of the microfiltrationmembrane. The resulting structure is an integral, monolithicframe-microfiltration membrane and may be used as such.

The resulting framed microfiltration membrane is soaked in a mixture of(all parts by volume except as indicated):

    ______________________________________                                        2-sulfoethyl methacrylate [1804-87-1]                                                                 30 parts                                              ethylene glycol dimethacrylate [97-90-5]                                                              16 parts                                              divinyl benzene (80%) [1321-74-0]                                                                     16 parts                                              dipropylene glycol      38 parts                                              azobis dimethyl isobutyrate                                                                           10 grams/liter                                        ______________________________________                                    

The microporous membrane of the saturated structure is thoroughlydrained and covered with Mylar film described in Example 1. It is heatedin an air recirculating oven at 85° C. for 4 hours, allowed to cool andstored in 2N sodium chloride solution yielding a framed CXM.

Similar results are obtained when poly(ethylene-co-ethyl acrylate) isused in place of ethylene-vinyl acetate copolymer. It is found thatother resilient polymers can also be used, such as EPM rubber,thermoplastic elastomers (e.g. Kraton 1101 styrene-butadiene blockcopolymers (SBS); Kraton 1107 styrene-isoprene block copolymers (SIS);polyurethane elastomers; Hytrel (TM duPont) copolyester-etherelastomers); Viton A(TM duPont) poly vinylidene fluoride-co-hexafluoropropylene; Viton B(TM duPont) poly(vinylidenefluoride-co-hexafluoropropylene-co-tetrafluoroethylene); Kel-F 3700(TM3M Co.) poly(vinylidene fluoride-co-chlorotrifluorethylene)poly(vinylidene fluoride-co-1-hydropentafluoropropylene). It is alsofound that crosslinked elastomers give similar results when they areused in place of ethylene-vinyl acetate copolymer. In such case theelastomer can be crosslinked in the mold in contact with the intendedmicroporous substrate for the membrane. The microporous substrate cansubsequently be saturated with a polymerizable liquid mixture which,upon polymerization will produce an IXM.

For some choices of elastomer and polymerizable liquid mixture, thecrosslinking of the elastomer and polymerization of the liquid mixturecan take place simultaneously. Such choices can be easily made by thoseskilled in the art. In general the elastomer should be capable ofcrosslinking at temperatures of about 100° C. or less over periods ofone to a few hours or be capable of being compounded to give suchcharacteristic. It is also obvious that the elastomer and thepolymerizable liquid mix should have limited mutual solubilities.

The polymerizable mixture recited in this example results in CXM. It maybe replaced with other appropriate polymerizable mixtures, for examplethat of Example 1 yielding in such case an AXM. The integral frame inthis example is roughly 3 mm thick including the margins of themicroporous-sheet-reinforced CXM which margins are impregnated withethylene-vinyl acetate copolymer (or other appropriate polymer as setforth above). By appropriate modification of the aluminum pattern ofexample I and/or by using the (positive) aluminum frame of Example 11 orin other appropriate ways obvious to those skilled in the art, thethickness of the integral frame can easily be made more or less thanroughly 3 mm including having a thickness about equal to that of the IXMor even less.

EXAMPLE 7

About 50 parts by weight of polyethylene are sheeted out on a rubbermill heated at about 110° C. There are then added about 150 partsAmberlite IRA 402(Rohm and Haas Co., Philadelphia, Pa.) in the form ofbeads, all of which pass through a U.S. Standard sieve No. 100. Themixture is milled at about 110° C. until the dispersion of the IRA 402in the polyethylene is uniform. The mixture is made into thin sheets.Two such thin sheets are pressed, one from each side, into apolyethylene screen under heat and pressure to make a reinforced,heterogeneous AXM. Other sheets are cut to the pattern of FIG. 1 andseveral of them inserted into the aluminum pattern of Example 1sufficient to overfill it slightly. The above reinforced membrane isplaced over the aluminum pattern and covered with a piece of the Mylarfilm described in Example 1. A steel plate is placed over the film andthe resulting structure heated until the membrane and the frame piecesin the aluminum pattern have fused together. After cooling the patternis placed in water and the composite membrane-frame removed from thepattern and stored in 2N sodium chloride solution.

It is found that similar composite membrane frames can be made when,instead of polyethylene, there are used: a mixture of about 42 parts ofpolyethylene and 8 parts of poly-isobutylene; NBR rubber compounded withsulfur, zinc oxide and a non-scorching accelerator; SBR rubber similarlycompounded; a copolymer of 95% vinyl chloride and 5% vinyl acetate; polychlorotrifluoro ethylene; a mixture of about 15 partspolytetrafluoroethylene and 35 parts polyethylene; linear low densitypolyethylene; or polypropylene. It is also found that similar compositemembrane-frames can be made when instead of Amberlite IRA 402 there aresubstituted other finely divided IXB such as Amberlite IRA 910, 410,458; Lewatit M-500 (Bayer AG, Leverkusen, BRD) or Purolite A-450(Purolite Co., Bala Cynwyd, Pa.). It is further found that similar,composite cation exchange membrane-frames can be made when instead ofAmberlite IRA 402 there are substituted finely divided CXB such asAmberlite IR 118 (Na) or 200; Lewatit S-100; or Purolite C-120E.

EXAMPLE 8

Composite frame membranes are made in accordance with Example 7 exceptthe frame portions are made from alternating layers of heterogeneous AXand CX sheet. The depleting bipolar junctions reduce by-pass current athigh cell pair potential drops.

EXAMPLE 9

Composite frame membranes are made in accordance with Example 7 exceptthe frame portions are made from sheet which has been loaded with 8%crosslinked polystyrene beads having a mesh size of 200 to 400 meshinstead of with Amberlite IRA 402. The resulting compositeframe-membranes reduce by-pass current at high potential drops per cellpair.

EXAMPLE 10

The frame portion of the aluminum pattern of Example 1 is filled with aroom-temperature-curing unsaturated polyester auto body putty. Amicrofiltration membrane sheet, as described in Example 6 is pressedinto the still soft putty. After the putty has hardened themicrofiltration membrane sheet is impregnated with the CXM mixture ofExample 6, drained, covered and cured as described therein. Thecomposite membrane-frame is removed from the aluminum pattern while thelatter is still warm. The resulting membrane is stored in 2N sodiumchloride solution.

It is found that similar composite membrane-frames can be made when theauto body putty is replaced with a two part epoxy loaded with micronicglass beads or with a resole type low temperature curing phenolformaldehyde copolymer loaded with micronic glass beads or when the CXMmixture of Example 6 is replaced with the AXM mixture of Example 1.

The frame portions of the composite membrane-frames of this example arequite hard in contrast to the more or less resilient frames of Example6. Such hard frame-membranes may be used, for example, for the dilutingcompartments of non-reversing ED or filled cell ED stacks since the hardframes define the geometry of the diluting cells more certainly. Thehard frames may be easily machined, if necessary, to give additionalprecision. When such hard frame-membranes are used for dilutingcompartments then resilient frame-membranes may be used for theconcentrating compartments.

The integral frame in this example is roughly 3 mm thick including themargins of the microporous-sheet-reinforced CXM which margins areimpregnated with the unsaturated polyester auto body putty (or otherappropriate polymer as set forth above). By appropriate modification ofthe aluminum pattern of Example I and/or by using the (positive)aluminum frame of Example II or in other appropriate ways obvious tothose skilled in the art, the thickness of the integral frame can easilybe made more or less than roughly 3 mm including having a thicknessabout equal to that of the IXM or even less.

IMPROVED METHODS OF FILLING ED STACKS WITH ION

Exchange Resin

A unit ED cell in which a CXM and an AXM are joined into a unit withoutframes has been described by Lacey et al., U.S. Office of Saline Water,Rep. 398 (1969); Lacey (page 15) and Nishiwaki (page 94) in "IndustrialProcessing with Membranes", eds. Lacey and Loeb, Wiley Interscience(1972); Garza et al., Proc. 5th Symp. on Fresh Water from the Sea, Vol.3 (1976) page 79. The sealed unit cells did not contain IX filling. Theywere operated without flow of solution through the cell. Such unit cellswere spaced apart between a single pair of electrodes and solution to bedesalted was passed through the spaces between the unit cells. Uponpassage of a direct electric current the unit cell behaved as aconcentrating cell and became filled with brine as a result of bothelectrodialysis and electo-osmotic water transfer. Depending upon thecharacteristics of the membranes, the brine could have a concentrationin the range of 10 to 20%. After some time of operation the current wasreversed and salts were transferred out of the (completely) sealed unitcells into the flowing stream between such unit cells. The apparatus hadthe advantage of a single stream (instead of the two streams in normalED or filled cell ED). It has the disadvantage of dis-continuousproduction of desalted product (unless more than one ED stack is used)and of serious scale formation in the non-flowing sealed unit cells. Theprocess has not become commercial.

Desalination 24 (1978) pages 313-319 and 46 (1983) pages 291-298disclose a modification in which the sealed unit cell has a concentratestream outlet (but not a concentrate stream inlet). The unit cells arespaced between a single pair of electrodes as described above andoperated continuously without current reversal. The spaces between theunit cells could be filled with knitted ion exchange net or by pouringin IXB. The unit cells contained thin spacers which however produced noturbulence promotion at the low concentrate effluent rate. Scaling wasan even more serious problem since there was no current reversal toameliorate such scaling.

In none of the above references did the unit cell contain IX particlesor fabric.

EXAMPLE 11

Aluminum sheet about 3 mm (0.12 inches) thick is machined to a(positive) frame as shown in FIG. 1 except the outside dimensions aredecreased by about 1 mm on each outside edge and the cavities areincreased by about 1 mm all around so the resulting aluminum frame fitsloosely into the aluminum pattern of example 1. All the edges arerounded. The aluminum frame is coated with Teflon. The aluminum patternof Example 1 and the coated aluminum frame are heated. A piece of typePE-C-1 CXM(homogeneous, polyethylene based CXM available from theInstitutes for Applied Research, Ben Gurion University of the Negev.,Beersheva, Israel) is placed over the hot aluminum pattern and the hot,coated frame slowly pushed into the pattern, deforming the CXM into apocket. The assembly is allowed to cool and the coated aluminum frameremoved. The pocket membrane is removed from the pattern and holes cutcorresponding to the internal manifolds. The membrane is re-insertedinto the pattern. The areas near and including the entrance and exitchannels and corresponding manifold holes are filled with non-wovenscreen as described in Example 3. The remaining cavity in the pocket isfilled with a mixture of equal volumes of Dowex Marathon C cationexchange granules and Dowex Monosphere 550A anion exchange granules.("Dowex", "Marathon" and "Monosphere" are trademarks of Dow ChemicalCo., Midland, Mich.). A piece of PE-A-1 AXM also available from theabove mentioned Institutes for Applied Research) is placed over thefilled CXM pocket. The AXM is covered by a piece of the Mylar filmdescribed in Example 1 and the resulting structure covered by a heavysteel plate. The whole resulting structure is heated in a recirculatingair oven until the surfaces of AXM and CXM which contact each other fusetogether. After cooling the sealed, filled pocket is removed from thepattern, care being taken to keep it horizontal.

The resulting structure is essentially that represented schematically inFIGS. 2 and 4. In FIG. 2, 21a and 21b represent the pockets formed bypressing one of the membranes into the pattern of FIG. 1. 23a, 23b, 25a,and 25b represent non-woven screen which confine the IXB (29a, 29b, 29'aand 29'b) to the pockets 21a and 21b and help to distribute fluid evenlythrough the IXB. Although 29a and 29'a respectively 29b and 29'b areshown for convenience as separated regions, it will be understood that29a and 29'a respectively 29b and 29'b are contiguous, continuousregions from 23a to 25a and from 23b to 25b. 28 (repeated several times)represents the areas in which the AXM and CXM are joined together. InFIG. 2 such areas are shown as extending to the edges of the pattern 10of FIG. 1. It will be clear however that it is only necessary that thejoined areas be such that fluid passing from manifold aperture 22a tomanifold aperture 24a and from manifold aperture 22b to manifoldaperture 24b be confined to the interior of the pockets 21a and 21b.

22a, 24a, 22b and 24b represent the entrance and exit manifold apertureswith which the pockets 21a and 21b are in communication. 27a, 27'a, 27band 27'b represent manifold apertures for fluid streams by-passingpockets 21a and 21b.

Pockets, such as 21a and 21b, may also be referred to as pillows, packs,packets, purses, sacs, sacks, bags and other synonyms.

FIG. 4 is a schematic representation of a side view of the pillow(s) ofFIG. 2 through section a--a. Like features in FIGS. 2 and 4 have thesame numbers.

EXAMPLE 12

Hard frames, are made as in Example 10 and resilient frames as inExample 6 in each case omitting the fabric intended for IXM substrate. Afive cell pair filled cell ED stack is assembled using hard frames fordilute spaces. Sealed, filled pockets made in accordance with Example 11are placed in the dilute spaces. The concentrate spaces are filled withnon-woven mesh. The electrodes and electrode spaces are as described inExample 3. The CXM of the sealed, filled pockets face the cathode. Thestack is clamped together and operated as described in Example 3. Thereduction in conductivity of the diluting stream is about 90%.

Note that in the stack of this example the only membranes are those fromwhich the sealed, filled pockets (pillows) are fabricated.

At the conclusion of the test, the stack is disassembled. No significantamount of slumping of the IXB is observed and the sealed, filled pocketscan be removed and replaced easily. Since loose IXB must not be removedfrom the stack during disassembly, such disassembly and reassembly areeasy and clean.

A stack is also assembled using sealed pockets made as in Example 11 butusing unreinforced film made as in Example 7. Similar performance isobtained.

A stack is also assembled in which sealed, filled pockets are alsoplaced in the concentrate spaces, the CXM portion of such pocketscontacting the CXM portion of the pockets in the dilute spaces. The AXMportion of the pockets in concentrate spaces contact the AXM portion ofthe pockets in the dilute spaces. The stack is operated in a 24 hourreversing type mode.

It is also found that suitable sealed, filled pockets can be made usingcontact adhesive, assembling first one end of the pocket around porousinsert at such end, continuing the seal toward the other end, pouring ina measured, predetermined volume of appropriate IXB, inserting a secondporous insert (in the areas near and including the channels andcorresponding manifold holes) and finishing the seal. The resulting,sealed, filled pocket (pillow) is placed on a horizontal flat surfaceand the IXB smoothed out uniformly. In such case, the AXM and/or the CXMcan be molded before assembly into a pocket in order to give a smootherfit into the frames. If both (or neither) AXM and CXM are molded thenthe appropriate spacer can be made of two plies, the edges of thepockets inserted between the plies. One ply can be resilient and theother hard. A single ply or a single frame can be hard in the interiorand resilient on one or both surfaces. Although it defeats some of theadvantages of the system illustrated in this example, the sealed, filledpockets can be bonded to the appropriate frame or frames or to theappropriate frame ply or plies for example by thermal welding, adhesiveor non-corroding staples.

The frame of FIG. 1 contains two cavities. Individual sealed, filledpockets (bags) can be made for each cavity or (Siamese) twin pockets canbe made. Larger apparatus may contain more than two cavities in whichcase also individual pockets can be made for each cavity, twin pocketsfor adjacent cavities or multiple parallel pockets for the frame as awhole.

In this example, the filling in the sealed, filled pockets has beenconventional gel or macroporous beads. However short-diffusion-path("SDP") beads may also be used. Such beads have central core which isnot ion-exchange active, which core may consist of the same basispolymer as the exterior of the bead or of another polymer or may beinorganic. SPD beads may also be hollow. The filling need not be sphereshaped but can be granular, fibrous, cylindrical or have any otherdesired shape. The pocket can be filled with a single layer or multiplelayers of long rods (filaments) parallel to the direction of flow. Manynon-spherical shapes are easily made by extruding blends ofpolyethylene, styrene (or chloromethyl styrene or vinyl pyridine), anddivinyl benzene and subsequently curing and activating or by similarextrusion of blends of polyethylene, poly styrene (or poly chloromethylstyrene or poly vinyl pyridine) and subsequently activating as disclosedby Govindan et al. (e.g. Indian J. of Technology 13, February 1975,pages 76-79). Membranes suitable for making sealed, filled pockets(pillows) can also be made by such technology. Such interpolymers mayalso be used as binders for micronic IXB producing thereby improved(quasi-homogeneous) heterogeneous IXM.

EXAMPLE 13

A polypropylene/polyethylene non-woven fabric (FO 2450/025 from CarlFreudenberg) is swollen in a mixture of 95 parts of styrene, 4 parts of80% divinyl benzene and 1 part of benzoyl peroxide at 60° C. for 20minutes. The excess liquid is then drained off, subsequently blotted offwith paper towelling (alternatively blown off with clean air) and theresulting fabric kept in an aqueous saturated solution of sodium sulfateat 70° C. for 6 hours. The resulting fabric is rinsed with water, driedand kept for 5 hours in a mixture of 75 parts of chlorosulfonic acid and25 parts of carbon tetrachloride, resulting after rinsing and hydrolysisin a non-woven CX fabric. Instead of chlorosulfonic acid, the styrenatedfabric may be kept in 98.3% sulfuric acid at 100° C. for 9 hours. Fourgrams of mercuric chloride per liter are used as a catalyst.

A similar styrenated non-woven fabric is kept in refluxing chloromethylether for 4 hours using tin tetrachloride as a catalyst. Thechloromethylated fabric is soaked first in methanol and then for 24hours at room temperature in a 25% solution of trimethyl amine inacetone.

EXAMPLE 14

The non-woven fabric of Example 13 is swollen in a mixture of 95 partsof vinyl benzyl chloride, 4 parts of 80% divinyl benzene and 1 part ofbenzoyl peroxide. Excess liquid is drained and blotted off, the swollenfabric is sandwiched between Mylar film (described in Example 1) and theresulting sandwich placed between glass plates. It is cured at 70° C.for 6 hours in a recirculating hot air oven. The resulting "chloromethylstyrenated" fabric is immersed for 24 hours at room temperature in a 25%solution of trimethyl amine in acetone.

EXAMPLE 15

The non-woven fabric of Example 13 is swollen in a mixture of 95 partsof 2-methyl-5-vinyl pyridine, 4 parts of 80% divinyl benzene and 1 partof benzoyl peroxide. The resulting swollen fabric is drained, blottedand cured as described in Example 13. It is then immersed for 24 hoursat 25° C. in a solution composed of 2 parts of methyl iodide and 8 partsof methanol and then thoroughly rinsed with hydrochloric acid.

EXAMPLE 16

Sealed, filled bags (pillows) are made as described in Examples 11 and12, using for each bag a CX porous fabric according to Example 13 and anAX porous fabric according to Examples 13, 14 and 15. The apparatus ofExample 3 is disassembled and the loose IXB replaced with the sealed,filled bags of this example, care being taken that in each case the CXporous fabric of the bag is in contact with a CXM and the AX porousfabric is in contact with an AXM. The resulting stack is operated asdescribed in Example 3, obtaining approximately the same performance.

It is found that, since the IX fabrics are porous, simplified bags canbe made which bags occupy only the cavity in the frames and do notextend into the entrance channels. In this case the bags do not containnon-woven screen near the entrance and exit channels.

The resulting structure is essentially that represented schematically inFIGS. 3 and 5. In FIG. 3, 31a and 31b represent the pockets (pillows)having porous casings as in this example. 38 (repeated several times)represents the frame into which the pillows are inserted. IXB arerepresented by 39a, 39'a, 39b and 39'b. Although 39a and 39'arespectively 39b and 39'b are shown for convenience as separatedregions, it will be understood that 39a and 39'a respectively 39b and39'b are contiguous, continuous regions from one end of the pillows 31aand 31b to the other end. 32a, 34a, 32b and 34b represent the entranceand exit manifold apertures with which the pillows 31a and 31b are incommunication through the porous casing of the pillows. 37a, 37'a, 37band 37'b represent manifold apertures for fluid streams by passing frame38.

FIG. 5 is a schematic representation in side view of a pillow of FIG. 3through section b--b. Like features in FIGS. 3 and 5 have the samenumber. 33a and 33'a represent the porous ion exchanging casing ofpillow 31a.

EXAMPLE 17

Hybrid sealed, filled bags (pillows) are made using in each case eitherAX porous fabric and CXM or AXM and CX porous fabric. The stack ofExample 12 is disassembled and the sealed, filled pockets from thedilute spaces and non-woven mesh from the concentrate spaces replacedwith an alternating sequence of the hybrid bags of this example, makingsure that the CXM portion of a bag is on the cathode side of each dilutespace and the AXM portion (of bags having AXM) is on the cathode side ofeach concentrate space. In such case such CXM portion will contact theporous CX fabric of the adjacent bag and each AXM portion will contactthe porous AX fabric of the bag adjacent to it. It will be clear, thatin this example, separate IXM are not required. The bags of this exampleeliminate any problems with membrane-to-membrane contact which may befound when the sealed, filled pockets of Example 12 are used in bothdilute and concentrate spaces. The stacks may be easily assembled,disassembled, inspected and reassembled in the field. In such case thereseems to be some advantage to using separate pillows for each cavity inthe frame.

EXAMPLE 18

Square pillows are prepared having a width and a height equal to thewidth of the cavity of Example 12. One side of the pillows consists ofporous CX fabric prepared according to Example 13 and the other side ofporous AX fabric prepared according to Examples 13, 14 or 15. Thefabrics are first formed into a pocket by heat welding, then apredetermined volume of the mixed IXB used in Example 11 is put in thepocket. The open end of the pocket is then heat sealed to make a pillow.The edges of the pillows beyond the heat seals are trimmed off. Thevolume of IXB used is predetermined so that when the pillows are placededge-by-edge in the cavities of Example 12 and gently flattened, thethickness of the pillows is only slightly thicker than the thickness ofthe cavities. The apparatus of Example 16 is disassembled and thesealed, filled bags replaced by pillows of this example, edge-to-edge,taking care that in every case the CX porous fabric of the pillow is incontact with a CXM and the AX porous fabric is in contact with an AXMand the pillows are pushed together so there appears to be littleopportunity for by-pass flow around the pillows. The resulting stack isoperated as described in Example 3, obtaining approximately the sameperformance.

It is found there is some advantage to dyeing one of the porous IXfabrics a distinctive color before assembly of the pillows, theadvantage accruing from the ease in determining which side of the pillowis AX and which CX during assembly of the stack.

Disassembly and reassembly of the stack is found to be rapid and clean.

EXAMPLE 19

Square packets (pillows) are prepared as in Example 18 except eachpacket consists either of porous CX fabric and CXB or of porous AXfabric and AXB. The stack of Example 18 is disassembled and the packetsreplaced with the packets of this example, the packets alternatingedge-by-edge CX by AX, the closest packet to the entrance channel beinga CX pocket. The apparatus is operated at the same flow rate and voltageper cell pair as the apparatus of Example 18. Performance isapproximately the same.

EXAMPLE 20

The non-woven fabric of Example 13 is immersed in a mixture of sulfurylchloride and carbon tetrachloride (about 3:1 by volume) in the presenceof 0.25% (w/w) of anhydrous aluminum chloride and kept at about 35° C.for about 8 hours in daylight. The fabric is then washed in carbontetrachloride and hydrolyzed for about 1 hour in 5% (w/w) sodiumhydroxide in water to make a CX fabric.

EXAMPLE 21

The non-woven fabric of Example 13 is immersed overnight in decalin.After draining it is illuminated in a glass walled tank in a 1:2 mixtureof Cl₂ :SO₂ with incandescent lamps for 5 hours. One part is hydrolyzedin 10% sodium hydroxide at 60° C. for 4 hours giving a porous CX fabric.Another part is reacted with N,N-dimethylamino-3-aminopropane at roomtemperature for 2 days and finally treated in a 20% solution of methylbromide in alcohol yielding a porous AX fabric. Packets (pillows) areprepared from the porous AX or CX fabrics of this Example according tothe process used in Example 19. The pockets are found to give about thesame performance as the pockets of Example 19.

The structures of Examples 18, 19, 20 and 21 are essentially thoserepresented schematically in FIG. 6. In FIG. 6, 61a, 61'a, 61b and 61'brepresent pillows (packets, bags, pockets) having porous enclosures(casings, cases) as in these examples. (16 pillows in all arerepresented but only four are numbered). 68(repeated several times)represents the frame into which the pillows are inserted. (Only onelayer of pillows is shown. It is clear that more than one layer may beused and the arrangement of pillows in one layer may be off-set(staggered) with respect to those in another, e.g. adjacent layer. 62a,64a, 62b and 64b represent the entrance and exit manifold apertures withwhich the cavities in frame 68 and the pillows are in communication, thelatter through the porous envelopes of the pillows. 64a, 67'a, 67b and67'b represent manifold apertures for fluid streams by-passing frame 68.

In Examples 18, 19, 20 and 21 the pillows are roughly square and flat,similar in construction to tea bags. It is clear that they may haveother shapes, also useful, according to this invention, e.g. rightparallelepipeds with the long dimension perpendicular or parallel to thegeneral direction of flow of fluid; bodies having triangularcross-sections, fitted closely together by rotating each body 180° withrespect to similar adjacent bodies. Any shape or combination of shapesmay be used for the pillows which shape(s) permit(s) close packing ofsuch pillows.

EXAMPLE 22

A five cell pair filled cell ED stack is assembled using integral,monolithic framed AXM and CXM prepared in accordance with Example 6. Thethicknesses of the integral frames are about equal to those of the IXM.The end blocks are PVC into which has been milled recesses to acceptelectrodes. The electrodes are platinum electroplated titanium sheet. Onthe lower end block there are placed two ethylene-vinyl acetatecopolymer frames (not having integral IXM) each about 1 mm thick. Theopen space in such two ply electrode frame is filled with non-wovenscreen except that at the ends of such frame flexible urethane foam isplaced to block flow in the conduits to and from the internal manifolds.The urethane foam is slightly thicker than the two-ply electrode frame.Hydraulic connections to the flow paths of such electrode frame are madethrough the PVC end blocks and electrodes. The upper end block and upperelectrode space are similarly constructed.

The stack is assembled using for each concentrate space oneethylene-vinyl acetate copolymer frame (not having integral IXM) about Imm thick and similar to those frames used to make the two ply electrodeframes. For the dilute spaces three such frames are used. Duringassembly of the stack the concentrate spaces are filled with non-wovenscreens and the dilute spaces with a mixture of about 125 parts byvolume of Purolite Purofine A-300 and 85 parts by volume of PurolitePurofine C-100EF by spreading such mixture uniformly over the exposedintegral frame membrane. Alternatively the stack may first be assembled(with screen in the concentrate spaces) and the mixture of IXB pumped inas taught for example by U.S. Pat. Nos. 5,066,375; 5,120,416 and/or5,203,976. The stack is tightly clamped together by means of steel endplates and threaded tie-rods. The stack is used further to demineralizethe permeate from a reverse osmosis apparatus, which permeate has a pHaveraging about 6 and an electrical conductivity averaging about 1micro-Siemen/cm. The flow to each dilute space is about 10 ml/sec. Thefeed and outlet pressures of the concentrating and electrode streams areadjusted to minimize crossleaks. The feeds and effluents of the variousclasses of spaces are carried through coils of long pieces of flexibletubing in order to reduce by-pass currents. A d.c. current of about 0.5amperes is applied to the stack. It is found that the effluent from thediluting spaces has a conductivity averaging about 0.1 micro-Siemen/cm,i.e. about a 90% reduction in conductivity.

Similar results are obtained when the three-ply dilute frames arereplaced with hard frames about 3 mm thick made in accordance withExample 10 but omitting the microfiltration membrane sheet (and CXMmixture) or with resilient frames of about the same thickness made inaccordance with Example 6 also omitting the microfiltration membranesheet (and CXM mixture). Advantageously an integral, monolithicframe-AXM may be welded thermally or ultrasonically or by means of alaser to one face of the single or multiple ply dilute frame (orotherwise integrated therewith) by means of the frame of such AXM and anintegral, monolithic frame-CXM similarly integrated with the other faceof such dilute frame. If the dilute frame consists of more than one plythen such plies may also be welded or otherwise integrated together. Theresulting monolithic AXM-dilute-frame-CXM may be filled with appropriateIXB before completing the integration or afterwards, e.g. by pumping inthe IXB. Such monolithic AXM-dilute-frame-CXM containing IXB is a unitfilled dilute cell (pocket, pillow, bag etc.). A filled cellelectrodialysis stack made from such unit, filled, dilute cells is easyto assemble and disassemble but suffers from the disadvantage (comparedto the sealed, filled pockets of Example 12) that the dilute frames mustbe thrown away when one or both of the IXM must be replaced. Such unit,filled, dilute cells are however easy to make since they do not requirebonding of the IX resin of the integral frame IXM to the frame of thedilute space.

What is claimed is:
 1. An electrodialysis apparatus comprising dilutingspaces alternating with concentrating spaces, at least one dilutingspace defined by a frame, said frame comprising at least one cavitytherein, at least one fluid entrance manifold aperture, at least onefluid exit manifold aperture, at least one fluid entrance conduit havinga manifold end communicating with said entrance manifold aperture and acavity end communicating with said cavity, at least one fluid exitconduit having a manifold end communicating with said exit manifoldaperture and a cavity end communicating with said cavity, said cavitycontaining at least one pillow said pillow comprising a fluid permeablebody of ion exchange resin encased in ion exchanging sheet.
 2. Anapparatus according to claim 1 in which at least part of said ionexchanging sheet is permeable to bulk flow of fluid.
 3. An apparatusaccording to claim 1 in which at least part of said ion exchanging sheetis impermeable to bulk flow of water.
 4. An apparatus according to claim1 in which said pillow has at least in part a flat margin coextensive atleast in part with said frame.
 5. An apparatus according to claim 1 inwhich said cavity is juxtaposed to an anion exchange membrane on oneside of said cavity and to a cation exchange membrane on the other sideof said cavity and said pillow is juxtaposed to said anion exchangemembrane on one side of said pillow and to said cation exchange membraneon the other side of said pillow.
 6. An apparatus according to claim 5in which that part of the ion exchanging sheet of said pillow, whichpart is juxtaposed to said anion exchange membrane, has predominantlypositively charged, fixed, charged moieties.
 7. An apparatus accordingto claim 5 in which that part of the ion exchanging sheet of saidpillow, which part is juxtaposed to said cation exchange membrane, haspredominantly negatively charged, fixed, charged moieties.
 8. Apparatusaccording to claim 1 in which the interior of said pillow is incommunication with said entrance manifold aperture with respect to fluidflow.
 9. Apparatus according to claim 1 in which the interior of saidpillow is in communication with said exit manifold aperture with respectto fluid flow.
 10. Apparatus according to claim 1 in which said cavityis not juxtaposed to ion exchange membranes on both sides of saidcavity, said pillow has a flat margin coextensive at least in part withsaid frame, the interior of said pillow is in communication with saidentrance manifold aperture with respect to entrance fluid flow and incommunication with said exit manifold aperture with respect to exitfluid flow, the ion exchange sheet on one face of said pillow isimpermeable to bulk flow of water and has predominantly positivelycharged, fixed, charged moieties and the ion exchange sheet on the faceof said pillow opposite to said one face is impermeable to bulk flow ofwater and has predominantly negatively charged, fixed charged moieties.11. Apparatus according to claim 1 in which said cavity is juxtaposed toone ion exchange membrane having fixed charges of predominantly one signon one side of said cavity and is not juxtaposed to an ion exchangemembrane on the side of said cavity opposite to said one side, saidpillow has a flat margin coextensive at least in part with said frame,the interior of said pillow is in communication with said entrancemanifold aperture with respect to entrance fluid flow and incommunication with said exit manifold aperture with respect to exitfluid flow, the ion exchange sheet on one face of said pillow isjuxtaposed to said ion exchange membrane, the ion exchange sheet on theface of said pillow opposite to said one face is impermeable to bulkflow of water and has fixed charged moieties, the predominant amount ofsaid moieties having charges opposite to said one sign.